I'm a longstanding fan of Van Dijk. However, he is a CB at the end of the day, and I always think he's liable to have a handful of costly off days at the back where he loses focus on set pieces or gets caught out of position. The truly great CBs love defending. It's all they feel the need to do. I get the feeling that Van Dijk gets caught between two stools sometimes with his attacking instinct (not just contributing to set pieces, as per Terry, Kompany etc), and that isn't the best attitude for a player so obvioulsy tasked with organising his own team's defence.
The way Liverpool are set up right now to stay solid means he has to curb his attacking instincts. Last night was a surprise and maybe a calculated gamble to push him forward that almost came off. But Arsenal's attack had Liverpool on the ropes last night and should have capitalised, and I dont think there's any coincidence. He's the bedrock Liverpool are built on but he's not as stable at his core job as he should be.